The term electrical device as used herein includes devices having electrical inputs and/or outputs, devices having optical inputs and/or outputs, and devices having electro-optical inputs and/or outputs. Thus, although the description is directed to VCSELs the inventive concept is meant to be used with any electrical device.
Performance testing of semiconductor lasers is important during the wafer manufacturing phase in order to ascertain the lasers operability and its meeting particular specifications. It is advantageous to be able to validate the performance as early as possible so as to remove faulty wafers from further processing. A VCSEL semiconductor laser permits a first level of testing at the wafer level. VCSEL technology is described in a paper entitled "Progress in Planarized Vertical Cavity Surface Emitting Laser Devices and Arrays," by Morgan et al., SPIE, Vol. 1562, pp. 149-159, 1991, which is incorporated herein by reference.
A prime using candidate of VCSEL diode technology is data communication. In data communications, as in some other technologies, the high frequency characteristics of the diode laser are extremely important. The characteristics include rise and fall times, bandwidth, relaxation oscillation frequency, and small and large signal response. The wafer level testing techniques employed to date are not usable at high frequencies due to the long return path that the laser current has to travel. The severity of this problem is illustrated in FIG. 1. FIG. 1 shows a typical closed test loop for testing the performance of a laser on a wafer. The wafer diameter is typically three to four inches. It is noted that the maximum test frequency is inversely proportional to the physical length of the electrical circuit's wire return path. A long return path reduces the maximum test frequency. The return path starts at the point the wafer test probe 80 makes contact with the top diode contact 10, usually the anode. The laser diode 15 is located along the wafer cross section 25. The return path continues through the diode's active region 20, and out through the substrate 30. The substrate 30 is generally in contact with a chuck or wafer holder 40 which acts as a common ground contact. The loop is closed with a wire 50 connected to the test signal source 60 and returns through a second wire 70 attached to the probe 80. The test current must travel several inches through the electrical circuit's test loop. The long electrical wire represents a significant high impedance inductance that severely limits high frequency testing with the probe 80. The present invention is a method and apparatus to enable high frequency testing and overcome this limitation.